In an electrostatographic apparatus, an electrostatic image is formed on a dielectric material through photoconduction, electrostatic discharge or the like. A powder or liquid developer is applied to the material and adheres to the areas of high electrostatic charge to form a toner image. A toner image is transferred and fixed to a copy sheet to provide a permanent reproduction. Alternatively, the toner image may be fixed to the material on which the electrostatic image is originally formed. A powdered developer generally comprises two components which are mixed together. The image is actually developed by toner particles of small size which are colored black or another suitable color. Magnetic or non-magnetic carrier particles arc mixed with the toner particles to aid in application of the toner particles to the material carrying the electrostatic image.
The carrier particles are generally larger in size than the toner particles and exhibit different tribo-electric characteristics from the toner particles. Agitation of the developer causes the carrier particles and toner particles to rub together and produce opposite electrostatic charges which cause the toner particles to adhere to the carrier particles and also to the electrostatic image on the material for development. The carrier particles are not consumed in the development process, as are the toner particles, but are recovered and recycled.
The carrier particles which are recovered are mixed with the developer and used again in the developing process. However, since toner was consumed, the ratio of toner to carrier particles progressively decreases. For this reason, it is necessary to periodically add additional toner to maintain the toner to carrier ratio at the desired value. If this ratio chops significantly, the density of the developed image will drop by a corresponding amount. The ratio of toner to carrier particles in the developer is known in the art as the toner density.
However, factors other than toner density act to vary the developed image density. The amount of induced electrostatic charge in the developer, the shapes of the toner and carrier particles, the degree of adherence of the toner particles to the carrier particles, the amount of deteriorated carrier particles, the ambient temperature and humidity all act as variables to effect the image density.
The developing ability of the developer for electrostatography is thus the sum total of all variables including those enumerated above and may be defined as the amount of toner which adheres to the electrostatically charged surface of unit area and the unit of electrostatic charge. Due to deterioration of carrier particles, for example, the developing ability and thereby the image density may decrease even though the toner density remains constant or even increases.
Monitoring toner concentration in a two-component development system is a complex task, usually requiring measurement of representative samples of the developer mix over a period of time. The samples are best measured as the developer station is executing its primary role of developing the latent image, but the measuring technique should not interfere with the developer station's primary role. Optical monitors have been used to measure developer reflection as a function of toner concentration as shown in U.S. Pat. Nos. 4,956,668 and 4,266,141. These monitors work fine when there is a significant reflectance difference between the carrier and toner surface. When the carrier and toner have similar reflectance properties, another measure has to be used. With the small particle development (SPD) process, this situation exists. The toner and carrier are similar in reflectance. Magnetic permeability has been selected as the property to monitor. There is a direct correlation between magnetic permeability and the toner concentration.
Using magnetic permeability, sampling technique is crucial. The alignment and packing of the magnetic carrier particles effects the magnetic permeability along with toner concentration. A common location for a magnetic permeability toner monitor is at the bottom of the station sump. The developer is held near the monitor detecting zone by gravity and paddles in the sump transport the mix. This sampling technique is very much affected by the packing of the developer caused by gravity and the motion of the paddles. Spacing between the paddles and the monitors is another critical factor in sampling. Too tight of a spacing jams the mix against the monitor and too loose of a spacing does not transport the mix past the monitor. If the monitor is relocated up onto the side wall of the station sump, developer mix continuously flows by the monitor; however, air entrapment in the mix will give a false toner concentration reading.
A better situation would be a sampling technique that is not restrained by the sump configuration and the material is presented to the detecting monitor in a manner that toner concentration alone is affecting permeability. To be able to sample developer directly in the sump and not have to transport the mix to a sampling chamber would be a substantial benefit. To be able to consistently sample developer mixes possessing residual magnetism would also be a desired feature.